1. Field of the Invention
The present invention relates to a microphone the directivity of which has been made variable by making acoustic resistance variable.
2. Related Background of the Invention
In a single directivity condenser microphone having a front acoustic terminal and a rear acoustic terminal, it is possible to change its directivity by changing the acoustic resistance of the rear acoustic terminal. If the acoustic resistance of the rear acoustic terminal is reduced (lowered), more speed components of sound waves can be taken in and thereby the directivity becomes more bidirectional. If the acoustic resistance of the rear acoustic terminal is increased (raised), a heart-shaped curve indicative of the directivity characteristic changes into a hyper cardioid, a cardioid, and a sub-cardioid, and the microphone becomes more omnidirectional and if the acoustic resistance is extremely raised, the directivity becomes omnidirectional.
In the case of a single directivity dynamic microphone, it is possible to change its directivity by changing the acoustic resistance between a diaphragm and a rear air chamber. However, in order to obtain omnidirectivity, it is necessary to close the rear acoustic terminal.
As conventional examples of a variable directivity condenser microphone, there are examples described in Patent documents below. In Japanese Patent Laid-Open No. 2005-184347, an example is described, in which a variable directivity condenser microphone is configured by arranging insulating washers of a pair of condenser microphone elements back to back and coupling them via a coupling ring, and an elastic acoustic resistance material is disposed between both the insulating washers. In the assembly process, by adjusting the degree of tightening the respective insulating washers against the coupling ring, it is possible to change the amount of compression of the acoustic resistance material to change the acoustic resistance value and thus making the directivity variable. There is an advantage in that the acoustic resistance can be adjusted to obtain an excellent directivity by, for example, adjusting the degree of screwing one of the microphone elements against the coupling ring in a state in which the pair of condenser microphone elements continue to be coupled via the coupling ring.
Japanese Patent Laid-Open No. 7-143595 describes a variable directivity condenser microphone, in which a microphone unit is configured by coupling the two condenser microphone elements by a coupler with respective acoustic resistors facing each other, and the acoustic resistance material of the microphone unit constitutes a ladder-like acoustic circuit. There is an advantage in that a microphone having a stable characteristic on the whole can be provided easily by making an acoustic measurement of each element to confirm the tension of the diaphragm and variations in acoustic resistance before coupling the two elements by the coupler, and by combining elements having similar characteristics.
Japanese Patent Laid-Open No. 6-339192 describes a microphone unit accommodating a diaphragm, a back plate, a cover plate, etc. in a unit case, in which a path through which the back part of the back plate is communicated with the outside is formed and an adjustment mechanism that changes the acoustic impedance of the path is provided therein. As an example of the adjustment mechanism, an example is described, in which a path is provided in the cover plate, a bolt is screwed into the path, and the acoustic impedance of the path is changed by adjusting the degree of screwing the bolt.
The configurations to make the directivity of a microphone variable include a configuration in which a plurality of microphone units having different directivities are provided and the signal output of each unit is electrically added or subtracted, a configuration in which the distance between the front and rear acoustic terminals is made variable, etc., in addition to the various configurations described above. In addition, there is a variable directivity condenser microphone unit in which the polarity and voltage of a polarization voltage are changed.